#include <stdio.h>
int main(int argc, char** argv){
	int i		= 3;
	double ret	= 1;
	int flah = 1;
	for(; i<=900000000;){
		if(flah){
			ret -= 1.0/i;
		}else{
			ret += 1.0/i;
		}
		flah = (flah+1)%2;
		i += 2;
	}
	printf("%f", 4*ret);
	return 0;
}
